I'm thinking of somebody looking for a reservation at a nice place on short notice using OpenTable to figure out what's available. In that common situation, it could be helpful to have a list of the relatively few places that aren't on OT.

Yeah, that's an odd thing about the OpenTable system architecture, if the host system they install at the restaurant is off or has no network connection, nobody can make a reservation through the Web. I guess the upside is that if the restaurant's Internet connection goes down during service, they can still access their reservations.